A lithium-ion secondary battery is generally used with a protection circuit. Examples of main functions of the protection circuit for the lithium-ion secondary battery include controlling the lithium-ion secondary battery such that the lithium-ion secondary battery can be used under safe conditions, and preventing the lithium-ion secondary battery from being used if the lithium-ion secondary battery is no longer under safe conditions. To achieve these functions, the protection circuit of the lithium-ion secondary battery needs to be configured to safely stop functions of the lithium-ion secondary battery if the lithium-ion secondary battery is brought into a state that is out of its normal usage range (for example, an overcharge state or an overdischarge state).
Patent Literature 1 (PTL 1) discloses a protection circuit of a lithium-ion secondary battery. FIG. 9 shows an example of the protection circuit disclosed in Patent Literature 1. A protection circuit 100 shown in FIG. 9 includes a protection element 50, a detection-purpose IC 63, and an FET 61. The protection circuit 100 is connected to a battery pack 70 of a lithium-ion secondary battery. The battery pack 70 has a plurality of series-connected cells 71. The protection element 50, which is a fuse with a heating element, includes a heating resistor 53 and fuse elements 56. The protection element 50 is configured such that the fuse elements 56 are blown by heat that the heating resistor 53 generates when the heating resistor 53 is electrically conducted. The detection-purpose IC 63 is configured to detect a voltage across both ends of each cell 71. The detection-purpose IC 63 is configured to detect a voltage across both ends of the battery pack 70, and output a signal according to a detection result to a gate of the FET 61. When an overcurrent occurs in the battery pack 70, the fuse elements 56 are blown due to the overcurrent. When an overvoltage occurs in the battery pack 70, the voltage level of a detection signal outputted from the detection-purpose IC 63 is raised. As a result, the gate potential of the FET 61 rises, to bring the FET 61 into a switch-on state. A current flows between drain and source of the FET 61. A current flows in the heating resistor 53, so that the heating resistor 53 generates heat. As a consequence, the fuse elements 56 are blown. In this manner, the protection circuit 100 shown in FIG. 9 protects the lithium-ion secondary battery from overcharging.
Patent Literature 2 (PTL 2) discloses a protection circuit of a lithium-ion secondary battery. In the protection circuit shown in Patent Literature 2, a battery voltage is compared against a threshold value which is a recharging prevention voltage. This way, whether or not the battery voltage has reached a deep discharge region is determined. Patent Literature 2 discloses that recharging of a battery pack is disabled upon determination that the battery voltage has reached the deep discharge region.